1. Field of the Invention
The present invention relates to a wire employing an oxide superconductor and a method of preparing the same, and a stranded wire and a conductor employing such wires, and more particularly, it relates to a wire having a substantially circular sectional shape or a polygonal sectional shape which is substantially rotation-symmetrical and exhibiting a high critical current density, and structures of a stranded wire and a conductor having small ac loss employing the wire.
2. Description of the Background Art
Among conventional oxide superconducting silver sheath wires, a round wire having a circular section exhibits an extremely small critical current density as compared with a tape-shaped wire obtained through rolling. This is conceivably because the density of a superconductor is not increased in the wire due to absence of a rolling step in the process of preparing the round wire. This is also conceivably because superconducting crystals are not oriented by a lamellar structure which is specific to a bismuth superconductor due to the absence of the rolling step and hence c-axis orientation unsatisfactorily results from crystal growth in sintering.
As a result of development of long wires having critical current densities exceeding 10.sup.4 A/cm.sup.2, application of silver sheath bismuth oxide superconducting wires to superconducting power apparatuses with liquid nitrogen cooling is expected. Among such silver sheath bismuth oxide superconducting wires, however, only the so-called tape-shaped wire satisfies a practical critical current density, a long wire shape, a mass production technique and the like under the present circumstances. The tape-shaped wire is prepared by charging powder serving as raw material for a bismuth oxide superconductor in a silver pipe, drawing the same, engaging a plurality of such drawn silver pipes in a silver pipe for obtaining a multifilamentary wire, further drawing the wire, and thereafter rolling and heat treating the same.
Although a high capacitance conductor is experimentally prepared by spirally winding the tape-shaped wire on a cylindrical pipe for attaining a multilayer structure, high ac loss may be generated from the structure.
In ac application of a superconducting wire, ac loss resulting from a fluctuating magnetic field comes into question. In a conductor prepared by assembling superconducting wires, on the other hand, a problem of current drift arises from ununiform impedances between the wires. Due to such drift, ac loss generated in the conductor disadvantageously exceeds the sum of those generated in the respective wires forming the conductor. This problem takes place in a conductor obtained by superposing tape-shaped wires in a layered manner.
In a conventional conductor employing metal superconducting wires, impedances of filaments or the wires are uniformalized by twisting the filaments or the wires or through transposition of the filaments. Also in such a conductor employing oxide superconducting wires, it is important to employ round wires in which current drift is prevented by transposition of the superconductor itself, in order to reduce ac loss. In order to obtain a high critical current density in the oxide superconductor of ceramics, however, it has been necessary to reinforce grain bonding and increase crystal orientation by shaping the wire into a tape and sintering the same.
The oxide superconductor has a characteristic that the c-axis is oriented on an interface between the same and a metal, and its critical current density is improved with improvement of the c-axis orientation. The tape-shaped oxide superconducting wire has a high critical current density since its c-axis orientation is improved by pressing or rolling in working into a tape shape, and further the density of a superconducting crystal portion is increased. In case of preparing a round wire, however, only an extremely low critical current density has been attained as compared with the tape-shaped wire due to absence of the pressing or rolling step.
Japanese Patent Laying-Open No. 4-262308 (1992) discloses a round oxide superconducting wire having a section in which a metal, silver or a silver alloy and an oxide superconductor are alternately concentrically stacked with each other, for improving the critical current density. This gazette describes that c-axis orientation can be attained in this wire by alternately stacking the metal and the oxide superconductor for attaining a multiring structure and reducing the distance between the interfaces between the oxide superconductor and the metal, specifically to not more than 100 .mu.m between adjacent interfaces. Although this wire exhibits a critical current density which is higher by one order as compared with other conventional round wires, however, this critical current density value is smaller by one order as compared with the tape-shaped wire, i.e., by far lower than a practically required critical current density level.
Another example aiming at improving the critical current density of the round superconducting wire is disclosed in Cryogenics (1992) Vol. 32, No. 11, pp. 940 to 948. In the round wire described in the literature, 55 single-filamentary rods having rectangular sections are concentrically arranged in a silver tube in three layers. No critical current is decided as to the obtained wire in the literature. However, it is presumable that the critical current density of the wire described in the literature is not much high, as described later.